How to export mesh with normal map applied as OBJ or STL for 3D printing?
Normal maps are an "effect" added on top of your model, visually changing the apparent normal (direction) of the surface at a certain point to simulate additional details.

This simulation affects shading only, like lighting, reflections effects and self shadowing (not projected shadows though). There really isn't any geometry or details there only distorted directions, so unlike displacement or bump these can't efficiently be added back as geometry.

Bump maps on the other hand store "height" values, in a black and white image. While these are also computed to affect normals only, they could optionally be used to move the mesh surface up and down according to black or white values and thus build crude geometry details, in a process often called displacement.

This process is also inefficient and a rough approximation of the original, it requires high amounts of geometry for a inferior quality result.

Bottom line is there isn't as far as I know an efficient way to add back details.

You could try and use the normal map in a displacement node, using the option RGB to XYZ and see if you get any decent results. It will require a subdivision modifier before it with a high count of subdivisions, but expect numerous artifacts and low fidelity.